Fermenting and storing forage plants in an enclosed space or silo is an important way of providing fodder for domestic animals. However, natural fermentation is not always successful. This is thought to be due to the fact that the most important bacteria necessary for successful fermentation, i.e., Lactobacillus, Pediococcus, and Leuconostoc, are very scarce on the living undamaged plant tissue to produce silage. To improve the reliability of successful fermentation of forage plants, various silage additives have been proposed. Many of these additives, either alone or in combination, have been shown not only to enhance forage fermentation and reduce storage losses but to increase the food value of the forage plants. A review of various additives and their effect on silage can be found in The Biochemistry of Silage, Peter McDonald, pp. 129-67, John Wiley and sons (1981).
When used as silage inoculants, the various additives are referred to as "stimulants" and exert their effect by encouraging rapid lactic acid fermentation. One of the ways in which forage lactic acid fermentation can be stimulated is by the addition of microorganisms that increase the population of lactic acid-producing bacteria. The characteristics of ideal microbiological stimulants have been defined by Whittenbury (1961) Ph.D. thesis, University of Edinburgh, and include organisms that provide the maximum amount of lactic acid from sugars immediately available, grow vigorously and dominate other organisms, are acid tolerant to at least pH 4.0, are capable of fermenting sucrose, glucose, fructose, and pentose, are homofermentative (producing only lactic acid), do not decompose organic acids, possess a growth temperature range exceeding 50.degree. C., and possess little or no protolytic activity. Based on these criteria, Whittenbury suggested the pediococci and homofermentative lactobacilli would be the most suitable microorganism.
Pure cultures of Lactobacillus plantarum have been studied as silage inoculants and evaluated based on the Whittenbury criteria. Inoculation with L. plantarum of alfalfa, corn, sorghum, and wheat silages was reported by Ely, et al., J. Dairy Sci. 64:2378-2387 (1981). These authors reported that in small test silos, total facultative anaerobic microflora were increased in all silages by the addition of L. plantarum, and that the total numbers of lactobacilli were increased, abovt that of controls, in all but corn silages. In comparable studies, addition of L. acidophilus had no positive effect on the inoculated silages, demonstrating that the particular inoculant species is critically important. Rooke, J.A., Animal Feed Sci. and Tech., 13:269-279 (1985) reported similar results for silage inoculants containing L. plantarum added to perennial ryegrass. These results were in contradiction to those reported by McDonald (1981), leading Rooke, et al. to propose that different strains of L. plantarum may produce different results.
Moon, et al., U.S. Pat. No. 4,528,199, have disclosed a unique strain of lactobacillus, namely L. plantarum 2B, isolated from a corn silage fermentation that had not been inoculated with any supplemental bacteria. Inoculation of various forage materials with this strain were reported to produce superior results over a wide temperature range for all forages tested. The tests reported by Moon, et al.; Ely, et al.; and others were, however, conducted in experimental silos, where conditions were carefully controlled. These authors acknowledge that experience under normal farm conditions, where temperature and degree of anerobises are neither uniform nor carefully controlled, can produce results significantly different from those reported above.
Another problem with using a pure strain of L. plantarum as an inoculant is that this species has a relatively narrow pH growth curve and does not begin to produce lactic acid until the pH reaches about 5.0. For this reason, McDonald has suggested that the ideal inoculum should contain, in addition to L. plantarum, another species of bacteria that is active in the pH range of from 6.5 to 5.0, such as Streptococcus faecalis. Inoculation with a combination of these two bacteria resulted in a well preserved silage having a final pH of 4.1, with S. faecalis dominating during early stages of fermentation, finally giving way to the acid tolerant L. plantarum when the pH fell below 5.0.
An alternative solution to the problem is disclosed by Brown et al., U.S. Pat. No. 3,147,121. These inventors disclosed the use of Pediococcus pentosaceus (formerly cerevisiae) as a silage inoculant because this species produces lactate over a broad pH range. Inoculation of forage material with this bacteria produced satisfactory results in experimental test silos, but no data is provided under actual farm conditions.
Several authors have suggested a mixed culture of bacteria is preferable for use as a silage inoculant. A mixed culture of S. faecalis, L. plantarum and Leuconostoc mesenteroides has been demonstrated to be effective for inoculating ryegrass and clover (McDonald (1981)). Others have suggested that the safest approach for finding effective cultures is to isolate them from silage produced from the plants and under the conditions expected to occur where the inoculum is used, Lesins, et al., Can. J. Animal Sci. 48:15-25 (1968). Unfortunately, this solution to produce the best inoculation medium is not always possible. Furthermore, it is not clear that the best inoculant is one containing only the species that finally dominates the silage.
From the foregoing description of the prior art, it will be appreciated that a useful silage inoculant is needed, one that can effectively ensile a wide variety of forage material under the quite variable conditions found in farm silos. The inoculant should foster a timely succession of nonpathogenic microflora, culminating in dominating homofermentative lactate-producing organisms that are capable of quickly lowering the pH of the fermentable forage material to a pH suitable for preservation.